A transition is a passive microwave element making it possible to go from one means of propagation to another. It is thus possible to transmit a microwave signal through a system comprising waveguides of different shapes, e.g. rectangular and circular waveguides, microstrip lines, striplines, and/or coaxial cables.
It is frequently necessary to transmit a signal between a waveguide and a planar circuit. A ridge waveguide is a rectangular or circular waveguide provided with a metallic ridge. The planar circuit may be constituted by a microstrip circuit, by a coplanar circuit with or without a ground plane, or by a suspended microstrip circuit.
In a known manner, the transition may be of the type having localized constants or of the type having distributed constants:
A localized-constant transition between a waveguide and a planar circuit is shorter than the wavelength of the guided wave. It is usually constituted by a probe penetrating into the waveguide perpendicularly to the direction in which the waveguide extends, and connected to the planar circuit. The probe is constituted by the core of the coaxial cable or by an etched metal-plated line on a substrate whose opposite face is locally stripped of its metal plating. The drawback with that type of transition is that it requires the direction of the microwave signal to be changed through 90.degree., and the overall size of the transition is then large. This applies to E-plane and H-plane transitions. In addition, such transitions are difficult to implement and they do not offer a wide matching band.
A distributed-constant transition is no shorter than the wavelength of the guided wave. It is usually constituted by a smoothly-varying or stepped impedance transformer. That end of the impedance transformer which is situated at the transition has a ridge-shaped cross-section (see FIG. 5). That type of transition has a wider bandwidth. Reference may be made, for example, to French Patent Application No. 2 552 586 as applied to a transition between a waveguide and a coaxial line or a microstrip line.
The principle of that solution is described with reference to FIG. 1 which is a section view of a transition between a waveguide and a microstrip line, as described in the work "Microwave transition design" by J. S. and S. M. Izadian, Artech House 1988, Page 54, FIG. 4.1.
In FIG. 1, a waveguide 10 includes a cover 11 to which a ridge forming a smoothly-varying impedance transformer 12 is fixed. The ridge 12 is at the center of the waveguide 10, and its free end 13 is put in contact with a conductor 14 by putting the cover 11 in place, the conductor being mounted on a substrate 15 whose bottom face constitutes a ground plane. The conductor 14, the substrate 15 and the ground plane constitute a microstrip line. Electrical continuity is thus provided between the ridge 12 and the line 14.
The drawback with that solution is that it makes it necessary to comply with tight manufacturing tolerances in order for the electrical contact to be good. In addition, contact problems arise in the presence of thermal expansion.
A solution remedying that drawback consists in providing a flexible conductive link between the end of the ridge and the conductor provided on the planar circuit.
FIG. 2 is a section view of such a transition with reference numerals 10-15 identifying the same elements as like reference numerals already described in FIG. 1.
In a first configuration, the conductive link is referenced 20, and is represented by an uninterrupted line. The link 20 connects the end of the ridge 12 to the conductor 14 of the planar circuit, the contact points being referenced 21 and 22. In a second configuration, the conductive link is referenced 23 and is represented by a dashed line. The link 23 has contact points referenced 24 and 25.
The drawback suffered by those two configurations is that the links 20 and 23 cannot be put in place industrially (e.g. by means of a thermocompression machine) because the contact points 21 & 22 and 24 & 25 are not accessible from identical directions. By way of example, with respect to the link 20, a thermocompression machine that is to make the contact point 20 must have access in direction 26, while, to make the contact point 22, it must have access in direction 27. This means that two openings must be provided enabling access to be given, and that the transition must be turned over between two thermocompressions. Similarly, with respect to the link 23, the access directions for the thermocompression machine are 27 and 28, and the same problem arises.